Obsession

Defeat the Collector in the Tower of Love, in the far east of the City of Tears. First grab the Love Key from a corpse in the south-east of the Queen's Gardens, which unlocks the tower door. The Collector is a lanky hoarder who leaps around the room dropping jars full of the bugs and grubs he's caught - clear the enemies that spill out and punish him after his jumps. Beating him frees several grubs and marks any you've missed on the map, so it helps toward Metamorphosis.